The Long Term Evolution (LTE) project is evolution of a system of The Third Generation Telecommunication (T3G), improves and enhances a radio access technology of 3G, and uses an orthogonal frequency division multiplexing (OFDM) technology and multiple input multiple output (MIMO).
In the evolution of an LTE system, an important technology is use of a Small Cell (Small Cell). A network formed of Small Cells may greatly improve a system capacity. A Small cell technology has a relatively high requirement for synchronization between networks. Not only synchronization between base stations inside an operator network is required, but also relatively desirable synchronization between networks of different operators is also required. Synchronization includes time synchronization and frequency synchronization. Network synchronization between different operators that uses a time division duplex (TDD) technology helps to reduce frequency guard space required between different frequency bands, so as to increase frequency resources that can be used for transmission.
Frequency guard space refers to a particular frequency guard interval set between adjacent frequency bands used by operators. The spectral band is not used to transmit data and is only used to increase an interval between two adjacent frequency bands to reduce mutual interference between the adjacent frequency bands. Generally, according to research involving spectrums, a guard interval between 5 MHz and 20 MHz usually needs to be set between adjacent TDD frequency bands, to avoid interference between different frequency band signals.
Specifically, for a TDD system, such interference is mainly interference to downlink reception from uplink transmission. For example, user equipment of an operator A and another user equipment of an operator B are relatively close. In this case, if networks of the two operators are asynchronous, and when the user equipment of A is transmitting an uplink signal, the user equipment of B is receiving downlink data, and if a frequency interval between two frequency bands is excessively small, a case in which signal transmission of A interferes with reception of downlink data of B occurs. To avoid such mutual interference, during design of frequency bands, a part of spectrum is reserved as frequency guard space. The frequency guard space is not used for service transmission, and is only used to increase a frequency interval between an interference signal and a system that is interfered with, to achieve a guarding effect.
The foregoing discussion is continued. If transmission of A and reception of B can always take place alternately, an uplink of A always corresponds to an uplink of B, and a downlink of A always corresponds to a downlink of B. In this case, interference between systems of A and B may be avoided, and therefore frequency guard space may be omitted or only relatively small frequency guard space needs to be used. Such alternation in time may be implemented by means of synchronization between the networks of A and B. In this way, frequency guard space that is originally used for guarding may be used for data transmission, so that spectral efficiency and a system capacity can be improved.
Therefore, it is necessary to provide a solution to perform synchronization between different operator networks, so as to reduce frequency guard space by means of synchronization.